1. Field of the Invention
The present invention generally relates to joining an electronic component to a substrate. More particularly, the invention is concerned with an apparatus and method for dispensing a controlled quantity of molten solder thereto.
2. Related Art Statement
Reliable functioning of an electronic component is dependent upon the correct interplay between its member parts. For their inter-connection, such components still depend in the main upon soldering. In mass production, hundreds of millions of soldered joints are formed daily. Large computers and telephone systems have over 10.sup.5 soldered joints. Such joints are reliable if they have the correct design and a satisfactory process is available for forming the joints. Ideally, such joints should have a negligible contact resistance and an acceptable strength. Reliability calls for the joints not only to have the desired properties immediately after formation, but about 7 to 13 mm. This allows the component body size to be reduced somewhat. Surface-mounted components are soldered by applying a solder paste to the board through a fine mesh screen or thin metal stencil to control placement of the paste on the board. When a screen is used, selected spaces in the mesh are closed to create a pattern in the open spaces which corresponds to the pattern to be placed on the board. When a stencil is used, the pattern to be placed is etched directly through the metal. Either the screen or stencil is positioned over the board and solder paste is applied through the openings onto the underlying board by a squeegee. After the screen or stencil is removed, the desired pattern of paste is left on the board.
One limitation of the screen print method is that the surface of the board to which the paste is applied must be free of irregularities or projections. Also, the regions surrounding the area to be printed must be large enough to accommodate the frame that supports the screen. Similar disadvantages accrue when using a stencil.
From a thermal point of view, a reliable soldering process must be sufficiently hot to permit wetting, yet not be heated to a temperature at which the board or components to be soldered will be damaged. Ideally, the thermal characteristics should be such that the solder flowing to the area to be soldered does not cool so rapidly that solder flow is impaired before the joint has been effected.
U.S. Pat. No. 4,515,297 discloses methods for multi-point dispensing of viscous material. During a first time interval, pressure is applied to a dispenser to force paste out of dispensing orifices onto an underlying board. The dispensing tool is then moved upwards and away from the surface of the board at a constant velocity. During retrogression, the dispense pressure is constant before termination of the pressure. Thereafter, the dispensing nozzle continues to move away from the board, thus causing separation of the viscous material from the dispensing nozzle openings.